This invention relates to a four-stroke internal combustion engine and more particularly to an improved combustion chamber configuration for such engines.
In engine design, it is a well known practice to employ a stroke-to-bore ratio that is less than unity so that the bore is greater than the stroke of the engine. Such arrangements have a number of advantages. For example, such stroke-to-bore ratios provide large surface areas for a given engine displacement and permit the use of higher engine speeds. Because of the large surface area, it is possible to provide a greater valve area with engines having stroke-to-bore ratios less than unity.
In order to still further improve the valve area and to permit greater increases in engine speed, it has been the practice to employ multiple intake and/or exhaust valves for each combustion chamber. In addition to providing a stroke-to-bore ratio less than unity and preferably in the range of 0.8 or less, it is also desirable to provide a relatively high compression ratio for the engine. High compression ratios result in better engine performance both in the form of power output and fuel economy. It is desirable if the compression ratio can be maintained at 9 to 1 or more. However, the use of large valve areas makes it difficult, with conventional engine constructions, to maintain such high compression ratios.
The reason for this is that the lift of the valve should be related to its diameter and the larger the diameter the larger the lift to achieve good flow characteristics. However, in order to provide adequate clearance for such high valve lifts with large valves, it is necessary to dome the head of the piston and form recesses to clear the valves. This results in a combustion chamber that is long and narrow. If a single, centrally positioned spark plug is employed, such combustion chamber configurations require very long flame travel and it is difficult to insure complete combustion with such combustion chambers.
The effects of this may be seen in FIG. 1 of the drawings which is a cross-sectional view taken through the axis of a cylinder of an engine having a prior art construction. In this figure, a cylinder block is identified by the reference numeral 11 and has a cylinder bore 12 in which a piston 13 is supported for reciprocation. A cylinder head 14 is affixed to the cylinder block 11 and one or more intake valves 15 and exhaust valves 16 are carried by the cylinder head 14 and control the communication of intake and exhaust ports (not shown) with a combustion chamber 17 that is formed between the head of the piston 13, the cylinder bore 11 and the corresponding recess of the cylinder head 14. A spark plug 18 is provided with its gap positioned centrally of the combustion chamber 17.
As may be seen, the piston 13 is provided with a domed central portion in which recesses 19 and 21 are formed so as to clear the heads of the valves 15 and 16 and provide adequate clearance for their full lift. If a compression ratio of 9 to 1 or greater is to be employed with an engine of this type having a stroke-to-bore ratio of 0.8 to 1 or less, the combustion chamber will be long and narrow. Thus, it is difficult to insure complete combustion using only a single centrally positioned spark plug. In addition, this configuration of combustion chamber is prone to a condition known as "knocking".
It is, therefore, a principal object of this invention to provide an improved combustion chamber configuration for an internal combustion engine.
It is a further object of this invention to provide an improved combustion chamber configuration for an engine having a stroke-to-bore ratio of 0.8 to 1 or less and a compression ratio of 9 to 1 or greater.
It is a yet further object of this invention to provide a combustion chamber configuration that will provide good running, high outputs and good fuel economy through the use of a single centrally positioned spark plug and multiple valves.